What is it about Chicago Cubs prospects that always leaves us with an unsettled feeling in our stomachs, as though we’ve just spent a night partying wildly and, though we feel fine now, we know this is all going to come back to hurt us soon enough?

In theory, this should be an optimistic time for Cubs fans. With the emergence last season of shortstop Starlin Castro and outfielder Tyler Colvin, combined with the emergence this season of Darwin Barney, the words “young” and “promising” are being bandied about in regard to a team that just a couple of years ago seemed to be collapsing under the weight of age and contract.

Throw in the still young Matt Garza and catcher Geovany Soto, who just three years ago won the NL Rookie of the Year, and things should be looking up for the Baby Bears.

Yet there is just something unsettling about all of this.

Perhaps we remain uneasy because we have been here before. Before Starlin Castro and Darwin Barney, there was Jake Fox, the former Cubs prospect who hit .409 through the first 45 games of the 2009 Triple-A season before being called up to the Cubs full-time. Fox had also hit 17 home runs in those 45 games and just the year before hit 31 home runs in Double-A and Triple-A combined.

Of course, he never came close to that level of production with the Cubs, topping out at 11 home runs in 82 games with a .779 OPS (97 OPS+) before being shipped off to Oakland and then Baltimore.

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Remember Matt Murton? He hit .343 in 87 games between Double-A and Triple-A in 2005 and had a career minor-league average of .320 in six seasons, but he enjoyed just one merely solid full season with the Cubs, in 2006, when he went .297/.365/.444/.809 before prematurely fading in 2007 and being shipped to Oakland in 2008.

So it has gone for other Cubs prospects like Felix Pie, who batted .362 in 55 games at Triple-A Iowa in 2005, and Micah Hoffpauir, who hit .362 with 25 home runs and 100 RBI in Triple-A in 2008, and Eric Patterson, who hit .297 with an .817 OPS, 14 home runs and 24 stolen bases in 128 games in Triple-A in 2007, and Ronny Cedeno, who hit .355 with a .921 OPS and eight home runs and 11 stolen bases at Triple-A as a 22-year-old in 2005.

And let us not deign to mention Corey Patterson. Oh, Corey Patterson, the Hope against Hope, the Dream against Dream. This was a guy who showed superstar potential at every level, the ability to hit for power, to steal bases and potentially even to develop into an on-base threat. But his talent never materialized at the major-league level.

I guess we’ve just been here so many times before that it just gets harder and harder each year to believe it could possibly be for real. Indeed, it all seems to be crumbling down around us once again as we speak.

Tyler Colvin, mired in a season-long 7-for-62 slump, has been optioned back to Triple-A to see if he can regain his stroke.

Geovany Soto appears to be intent upon developing himself into the next great Every Other Year Talent, a la Juan Gonzalez, Alex Rodriguez, Justin Morneau and Prince Fielder, to name a few, though one would never mistake Soto’s “on” years for one of the on years of those just named. After resurging in 2010, Soto’s average is back into the .220s, his OPS back in the low .700s and his power once again swooning.

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Meanwhile, Barney and Castro are putting together preposterously similar empty-average seasons that seem deceptively good because they are both hitting over .300, but in reality each of these players has serious flaws.

For one thing, Barney would appear to be a creature of Wrigley Field; on the current road trip, Barney has seen his batting average drop from .345 to .315 in just seven games. Barney’s batting average is also deceptive: He does not get on base particularly well, he strikes out twice as often as he walks and he has absolutely no power (current slugging percentage: .394).

The bigger problem with Barney is that what he has done very early in his major-league career he never, ever did in his minor-league career; outside of a .317 average in 74 Double-A games in 2009, Barney never hit .300 in the minors, and his career minor-league OPS was .708.

For a second baseman, we’ll take it, but let’s not pretend this guy is Ryne Sandberg.

As for Castro, it is admittedly difficult to be critical of a 21-year-old guy who is currently leading the National League in hits with 62 and is batting .323. But since we’re here...

Castro is, at present, a disaster in the infield on defense, having already made nine errors after making 27 last season. He also appears to have virtually no range whatsoever, and he struggles in all of the major defensive metrics, including UZR and defensive runs saved.

On offense, Castro is somehow hitting 23 points higher than he did last season while posting the exact same on-base percentage, and he already has already grounded into seven double plays this season, which is halfway to last season’s total of 14.

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(I’ll go ahead and put this comment into parentheses, since it goes against the overall theme of what I’m trying to say here but bears noting nonetheless: In 16 games batting leadoff this season, Castro is hitting .397 with a .975 OPS and 41 total bases. In 16 games batting third, Castro is hitting .235 with a .576 OPS, and in 10 games batting second, he is hitting .256 with a .575 OPS. Can we get this guy into the leadoff spot permanently?)

At this point, obviously, being too critical of Castro would simply be too critical. Again, he is only 21 years old and is hopefully developing. At the same time, when you are looking at a franchise that has not developed a legitimate star out of its farm system since Mark Grace (for Pete’s sake), it is easy to be skeptical.

Frankly, while it is entirely possible that the Cubs may someday produce top-quality talent that the team can build around, it is looking increasingly like the Soto-Colvin-Castro-Barney combination ain’t it. But we shall see.

Now, if you will excuse me, I partied real hard last night, and something is beginning to not feel quite right.